And you really think you couldn't explain that to him? There's a large difference between what people intuitively expect and what they can understand if told about it. Envelope analogies can go surprisingly far. (Assuming they believe you. If they think you can't be trusted about such things, then that's a problem of course)
That's what you don't seem to comprehend. Yes, for you this seems obvious. But really, to the average Joe/Jill, this idea that you could get an email from the address you expect but it has been spoofed is gibberish.